A polarizer is a filter that has been treated to work as a grid for light. After being reflected by a surface, light rays arrive to the lenses with a particular orientation (polarized). By rotating
the polarizer ring you can control which reflections you want to remove from your footage. Be careful not to remove reflections that are essential to the story.
Polarizers are widely used in exterior shots to stop overcast skies from clipping. They also have the effect of increasing contrast and detail, so it is important to remember when shooting close objects or faces that they can reveal unwanted imperfections.